function natoms = NAtomsFluo(expp,cons,img_time,width,amp,width2d)

if nargin == 5
    total_num_electrons = amp*sqrt(2*pi)*width; % see p157 of Simon's lab book #1.
elseif nargin == 6
    total_num_electrons = amp*2*pi*width*width2d;
else
    error('Wrong # of arguments to NAtomsFluo')
end
total_photons = total_num_electrons/expp.coll_eff;
natoms = 2*total_photons/(img_time*2*pi*cons.Gamma); % numAtoms = (num_photons/time)/(scattering rate)
end